Why was a mod that adds saves based on high Wisdom score never made?

I believe in PnP you're supposed to receive a bonus to your saves(vs. spell?) based on high Wisdom scores. The feature even made it to the original Baldur's Gate manual(page 136 as "Mag Def Adj"), though it was never actually implemented in the game.

You'd think that, in the past 20 years, someone would make a mod for it... so I assume it's simply impossible to do alltogether?